1. Field of Invention
The current invention relates to magnetic resonance systems and more particularly to systems that provide adiabatic pulses.
2. Discussion of Related Art
Selective signal suppression is required in many Magnetic Resonance Imaging (MRI) and Magnetic Resonance Spectroscopy (MRS) scans. Usually the desired frequency-selection profile is made up of an excitation band with a constant effective flip-angle, flanked by transition bands that are as narrow as possible. Non-adiabatic pulses, such as the Shinnar-La Roux (SLR) pulses, can be specifically designed to achieve these goals, but desired profiles are achieved only in a narrow range of flip-angles (Pauly J, Leroux P, Nishimura D, Macovski A. Parameter Relations for the Shinnar-Leroux Selective Excitation Pulse Design Algorithm. IEEE Transactions on Medical Imaging 1991; 10(1):53-65). The frequency response profile of an SLR pulse designed for a 90° flip angle degrades when the flip angle is changed to 180°, meaning that a separate pulse shape has to be calculated. Depending on design parameters, sometimes SLR pulse profiles deteriorate even when flip angles are off by just ±30°. All non-adiabatic pulses suffer from similar variations in excitation profile at larger flip-angles. As a result, the use of these pulses for signal suppression require a careful calibration of RF power and RF excitation coil with a homogeneous RF field. With non-adiabatic pulses, a certain degree of on-resonance, B1-independent response may be achieved when used in optimized multi-pulse saturation sequences, but the off-resonance profile degrades as B1 values deviate from the design optimum. Thus, combinations of B1 and B0 inhomogeneity can seriously reduce the saturation efficiency. In high field magnets, this can be a serious problem, because localized shimming with higher-order shim coils will create more B0 inhomogeneity outside the volume-of-interest (VOI). The resulting insufficient suppression of signals from around the VOI is likely to lead to artifacts in most MR experiments, commonly referred to as outer volume signals. In short, the utility of these pulse sequences at B0 field strengths of 3 T and higher is often hampered by problems arising from power-deposition and B1 inhomogeneity.
The radiofrequency (RF) power deposition, or Specific Absorption Rate (SAR), is intricately linked with the B1 field inhomogeneity problem (Bottomley P A, Andrew E R. RF magnetic field penetration, phase shift and power dissipation in biological tissue: implications for NMR imaging. Phys Med Biol 1978; 23(4):630-643). The most obvious solution to problems arising from B1 inhomogeneity is to use either larger coils with more uniform B1 field, or adiabatic pulses that may be independent of B1 field. Either solution increases the SAR and the required RF amplifier power. Further, the solution of using a large coil does not address issues of outer volume signal suppression or susceptibility (e.g., metal implants in knees and necks of patients). Conversely, attempts to reduce SAR by using smaller excitation coils usually lead to loss of B1 field homogeneity.
Adiabatic pulses are amplitude- and frequency-modulated pulses that can achieve spatially uniform excitation, inversion, and/or refocusing when the transmitted RF power is over a threshold. Thus, adiabatic pulses can be insensitive to local B1 field variations (DeGraaf R A, Nicolay K. Adiabatic rf pulses: Applications to in vivo NMR. Concept Magnetic Res 1997; 9(4):247-268, Norris D G. Adiabatic radiofrequency pulse forms in biomedical nuclear magnetic resonance. Concept Magnetic Res 2002; 14(2):89-101). With adiabatic pulses, the desired frequency-selection profile (an excitation band with a constant effective flip-angle, flanked by transition bands that are as narrow as possible) can be achieved more effectively than with most non-adiabatic pulses. Selective adiabatic pulses may achieve an excitation profile with hardly any variation over multi-fold changes in the B1 field (Garwood M, DelaBarre L. The return of the frequency sweep: Designing adiabatic pulses for contemporary NMR. Journal of Magnetic Resonance 2001; 153(2):155-177).
However, to date, few attempts have been made to create multi-band adiabatic pulses (Goelman G. Two methods for peak RF power minimization of multiple inversion-band pulses. Magn Reson Med 1997; 37(5):658-665, Tsekos N V, Garwood M, Ugurbil K. Tagging of the magnetization with the transition zones of 360 degrees rotations generated by a tandem of two adiabatic DANTE inversion sequences. J Magn Reson 2002; 156(2):187-194, Tsekos N V, Garwood M, Merkle H, Xu Y, Wilke N, Ugurbil K. Myocardial Tagging with B-1 Insensitive Adiabatic Dante Inversion Sequences. Magnet Reson Med 1995; 34(3):395-401). Adiabatic pulses that invert more than two bands have been used for Hadamard-encoded localized spectroscopy (Goelman G. Hadamard encoding with surface coils for high SNR MR spectroscopy. Magn Reson Imaging 1999; 17(5): 777-781), and cardiac tagging. The multiple bands were achieved either by adding pulses after a frequency and time-shift (Goelman G. Fast Hadamard spectroscopic imaging techniques. J Magn Reson B 1994; 104(3):212-218), or by creating a DANTE pulse train. The DANTE pulse train tends to be longer, requiring a great deal of RF power for the excitation of many identical bands. These methods use adiabatic inversion pulses that may work with a non-homogeneous RF excitation coil, but these pulses typically deposit much more RF power in the sample than comparable non-adiabatic pulses. Thus, the use of these pulses can pose a problem when scanning biological samples and especially when scanning human subjects.
An adiabatic pulse can also be used at lower RF power, but then the effective flip angle may be no less B1-dependent than the flip angle of non-adiabatic pulses. This apparent loss of B1-independence may limit the usefulness of adiabatic pulses as B1-independent selective suppression pulses.
Therefore, there is a need in the art for improved adiabatic pulses for use with selective suppression of unwanted signals from more than one spectral locations in MRI or MRS applications.